The present invention relates generally to magnetic recording media used in rigid disc drives commonly used for computer data storage and methods of making the same.
In the hard disk drive industry, ever-increasing recording density demands continuous improvement in hard disk recording media so as to support a higher linear recording density (thousands of flux changes per inch-KFCI) and track density (thousands of tracks per inch-KTPI). Recording density is proportional to the product of KFCI and KTPI, and is typically expressed as giga-bits per square inch (Gb/in2). Currently the recording density is increasing at compound annual growth rate of approximately 100%.
In order for the media to be able to support high KFCI (e.g., over 250 KFCI), the pulse width (PW50-pulse width at 50% of pulse amplitude) needs to be as small as possible to reduce the inter-symbol interference so that high resolution at high recording density can be obtained. The resolution is defined as the pulse amplitude at high frequency divided by the pulse amplitude at low frequency. Based on generally known magnetic recording theory, in order to reduce PWso and hence increase resolution, the magnetic recording media must have high coercivity, Hc. For today""s typical recording density of 4 Gb/in2, the value of Hc needs to be on the order of 3000 Oe, and in future it needs to be greater for even higher recording density. Other means of reducing PW50 include increasing the hysteresis loop-squareness, generally defined as xe2x80x9cSxe2x80x9d which is ratio of remanent to saturation magnetization (Mr/Ms), increasing the coercivity squareness xe2x80x9cS*xe2x80x9d, increasing the remanent coercivity squareness xe2x80x9cS*remxe2x80x9d, and narrowing the switching field distribution (xe2x80x9cSFDxe2x80x9d), as described by William and Comstock in xe2x80x9cAn Analytical Model of the Write Process in Digital Magnetic Recording,xe2x80x9d A.I.P. Conference Proceedings on Magnetic Materials 5, p. 738 (1971).
Another factor which is important for increased KFCI and KTPI is that the signal to noise ratio (SNR) must be maximized. There are contributions to SNR from the electronics and the channel used to process the magnetic signal. But there is also intrinsic noise from the media that must be minimized. The largest contribution to the media noise is generated from the interparticle (or intercrystalline) magnetic exchange interaction. To suppress this exchange interaction, one must isolate the magnetic crystals from each other by one or more nonmagnetic elements (such as Cr atom) or compounds. The amount of separation need be only a few angstroms for there to be a significant reduction in intergranular exchange coupling. Another source of intrinsic media noise is the size or dimension of the magnetic grain. At recording densities of 4 Gb/in2 and greater, the bit size along the track is less than 0.1 xcexcm. Therefore to prevent the excessive noise arising from the physical dimensions of the grain, the diameter of each magnetic grain on the average should be less than approximately 0.01 xcexcm (10 nm) at this density, and even smaller for greater densities. Intrinsic media noise has been theoretically modeled by J. Zhu et al. in xe2x80x9cMicromagnetic Studies of Thin Metallic Filmsxe2x80x9d in Journal of Applied Physics, Vol. 63, No. 8, (1988) p. 3248-53 which is incorporated by reference herein. T. Chen et al. also describe the source of intrinsic media noise in xe2x80x9cPhysical Origin of Limits in the Performance of Thin-Film Longitudinal Recording Mediaxe2x80x9d in IEEE Transactions on Magnetic, Vol. 24, No. 6, (1988) p. 2700-05 which is also incorporated by reference herein.
The noise of the media can be reduced by decreasing the grain of the media, however smaller grain size may reduce Hc due to the onset of the superparamagnetic effect which comes about due to an inability of the grain to support the magnetization when it competes with the thermal fluctuation. In general, the onset of the superparamagnetic effect can be delayed by increasing the Ku of the magnetic grain through addition of platinum which has a high orbital moment, and also by improving the crystalline perfection of the hexagonal close packed (HCP) cobalt grains.
Therefore, an optimal thin film magnetic recording medium for high density recording applications that can support high bit density will require low noise and high signal without adversely sacrificing PW50, overwrite (OW) and total non-linear distortion (TNLD). Cobalt alloys which are currently used for optimization of certain of the above performance criteria typically include the addition of chromium (Cr), tantalum (Ta) and platinum (Pt), due to their ability to provide high Hc and high magnetic moment. Chromium is typically added in an amount greater than 10 atomic % to act as segregant to separate the cobalt alloy grains for noise reduction and for corrosion resistance. Other additives such as Ti, V, W, Mo, B and others are sometimes used. In all cases, the cobalt crystal structure must be hexagonal close-packed (HCP), and it is preferable to have the c-axis of the grains oriented in the film plane. This is accomplished by depositing a chromium film below the cobalt layer and arranging for the epitaxial growth of cobalt alloy grain above the chromium layer.
In order to describe the crystallography of the cobalt alloy and chromium, planes and directions in the crystal are denoted by generally accepted conventions, such as described in xe2x80x9cElements of X-ray Diffractionxe2x80x9d by B. D. Cullity published by Addison-Wesley Publishing Co. Inc., herein incorporated by reference. It is typical to describe the crystallographic planes and directions in hexagonal crystals such as cobalt by a 4 indices notation called Miller-Bravais indices, while cubic structure crystals such as chromium are denoted by 3 indices notation called Miller indices.
Brackets, xe2x80x9c less than   greater than xe2x80x9d are used to describe crystallographic directions, while parenthesis xe2x80x9c( )xe2x80x9d are used to denote specific lanes. xe2x80x9c{ }xe2x80x9d are used to denote a class of planes which are crystallographically equivalent. For example with chromium with body-centered cubic (BCC) crystallographic structure,  less than 001 greater than  direction is normal to a (001) plane. For a hexagonal crystal structure such as cobalt, the crystal surface with the most dense atomic packing is the (0001) plane and the direction normal to that plane is  less than 0001 greater than  direction. The  less than 0001 greater than  direction is often referred to as the c-axis as described earlier. The crystallographic directions and the surfaces for cobalt are shown in FIG. 1 and those for chromium are shown in FIG. 2.
The crystallographic orientation relationship that occurs between hexagonal cobalt film and BCC chromium film was originally reported by J. Daval and D. Randet in xe2x80x9cElectron microscopy on High-coercive-force Co-Cr Composite Filmsxe2x80x9d in IEEE Transaction on Magnetics, MAG-6, No. 4, (1970) p. 768-73. This work was preceded by work by J. P. Lazzari, I. Melnick and D. Randet in xe2x80x9cExperimental studies using in-contact recording on chromium-cobalt filmsxe2x80x9d in IEEE Transactions on Magnetics, Vol. MAG-5, No. 4, (1969) p. 955-59 where they reported that Hc of the cobalt film is increased by its deposition on top of a chromium underlayer.
The crystallographic orientation of chromium which promotes the cobalt c-axis to lie in the plane of the film is to arrange for chromium film to grow with  less than 001 greater than  preferred growth, which means that {001} type planes of chromium lie in the plane of the film. It has been found that the atomic spacing of cobalt {11{overscore (2)}0} type planes matches reasonably well with the atomic spacing of a {001}cr plane as shown in FIG. 3 hereof. Lattice spacings for pure Cr (ao=2.885 xc3x85) and pure Co (c=4.069 xc3x85, ao=2.507 xc3x85) are illustrated in FIG. 3. As seen in the Figure, the  less than 0001 greater than  direction of cobalt is aligned with the  less than 110 greater than  direction of the chromium lattice in the plane of epitaxy. In this direction, the Cr and Co lattices are closely matched and the mismatch is around 0.3%. Along the orthogonal direction ( less than 01{overscore (1)}0 greater than Co); the mismatch with the Cr lattice is much larger at around 6.4%. In this orientation relationship between cobalt and chromium, the lattice match is close only in one direction. The same holds true for alloys of cobalt. It should also be pointed out that in the above orientation relationship between chromium and cobalt, there are two equally plausible configuration for the cobalt. The  less than 0001 greater than Co direction can lie along two orthogonal  less than 110 greater than Cr type directions. In fact when the chromium grains are large, two variants of cobalt grains which are oriented 90xc2x0 to each other can form on the {001} surface of the chromium grains as described in xe2x80x9cEffect of Microstructural Features on Media Noise in Longitudinal Recording Mediaxe2x80x9d by T. Nolan et al. published in Journal of Applied Physics, 73(10), May 15, 1994 p. 5566-68.
A large variety of cobalt alloys have been used with a Cr undercoat. In its current industrial form, the Cr undercoat thickness is typically between 50 to 2000 xc3x85, and it is deposited on a heated substrate. A high degree of epitaxy between the Cr and the magnetic layer is required in order to obtain high Hc and high hysteresis loop squareness. Typically, Cr grows with strong  less than 100 greater than  orientation at high temperature, e.g. near or above approximately 2000xc2x0 C. In the plane of the film, the epitaxial relationship is  less than 110 greater than Cr// less than 0001 greater than Co, and {100}Cr//{11{overscore (2)}0}Co where xe2x80x9c//xe2x80x9d denotes xe2x80x9cparallel toxe2x80x9d. Alloying elements can be added to either chromium and cobalt or to both to attempt to match the lattice better. As mentioned before however, only one direction along the crystallographic direction can be very closely matched, while the other direction (orthogonal) will always be mismatched (e.g. over about 5%) in the above epitaxial orientation.
More recently, it has been shown by K. Hono, B. Wong, and D. E. Laughlin in the article xe2x80x9cCrystallography of Co/Cr Bilayer Magnetic Thin Filmsxe2x80x9d in Journal of Applied Physics 68(9) (1990) p.4734-40 that in-plane c-axis orientation may be achieved through other crystallographic relationship between Cr and Co lattice. The following lattice plane relationships have been proposed: (002)Cr//(11{overscore (2)}0)Co, (110)Cr//(10{overscore (1)}1)Co, (110)Cr// (10{overscore (1)}0)Co, and (211)Cr//(10{overscore (1)}0)Co. Generally, the addition of alloying elements into cobalt expands the lattice. For an alloy composition of CoCr10Pt18 for example, the lattice parameters are calculated to be Co=4.148 xc3x85, ao=2.556 xc3x85. For this composition, the lattice mismatch for pure Cr and CoCr10Pt18 alloy for several combination of planes are listed in Table 1. The two mismatch numbers are two orthogonal directions in the plane of the epitaxy. It can be seen from table 1 that best epitaxial match can be obtained between (110)Cr and (10{overscore (1)}1)Co for a lattice mismatch of 0.2% and 2% respectively. However, in this case the c-axis of the cobalt is tilted 28xc2x0 out of the plane. Another closely matched relationship is (211)Cr and (10{overscore (1)}0)Co for a lattice mismatch of 1.7% and 2% respectively. For other epitaxial relationships where the c-axis lies in the plane of the film, the difference in mismatch along the two directions is always greater.
Since the original work by J. Daval and D. Randet on Co/Cr epitaxial film structure, there are many examples of work on both the cobalt and chromium underlayer alloys to improve the recording performance of the cobalt/chromium alloy structure. A variety of schemes have been proposed to improve the lattice matching between Cr or Cr alloys with the cobalt alloy, and hence improve the in-plane orientation of the cobalt, and improve Hc and other properties as previously noted. There are several approaches. The first involves alloy or deposition variations on a basic two layer structure, involving Cr and Co alloy films. The second approach involves use of multiple layers in the undercoat or a different material other than Cr or alloys of Cr in an attempt to affect the magnetic properties. Thirdly, multiple magnetic layers can be used to attempt a better in-plane orientation. These approaches are described in copending patent application Ser. No. 08/984,753, now U.S. Pat. No. 6,150,015, which application is assigned to the assignee of the present invention and which application is incorporated by reference herein.
In the prior art, a common theme is that lattice matching between Co alloy and Cr is claimed as the key contributor to high Hc and lower noise. Particularly with respect to double magnetic layers using CoCrTa first layer and CoCrPt second layer, it is claimed that CoCrTa is better lattice matched to Cr, whether the orientation relationship claimed is (200)Cr//(11{overscore (2)}0)Co, (110)Cr//(10{overscore (1)}0)Co or sometimes even (110)Cr//(10{overscore (1)}1)Co. However, as shown earlier in table 1, a perfect lattice match in all directions is not possible. Furthermore, perfect or even good epitaxy in actual production machines is also difficult to achieve. One reason is that residual gases in the system which are often water vapor and oxygen which can be produced from the water vapor, can affect the interface between the Cr and the Co alloy layer and cause loss or partial loss of epitaxy.
Therefore, under deposition conditions that are not entirely perfect in terms of vacuum conditions and at high rate of deposition typically used in a commercial application, obtaining good epitaxy between Cr and Co-alloy layer is difficult to achieve. Hence c-axis in-plane orientation of the hexagonal cobalt film is not necessarily achieved, leading to lower Hc than is possible for a given Pt content of CoCrPt based alloy, and also reduced squareness. In addition, the grain size may not be uniform and the grains themselves may not have the proper crystalline perfection for high anisotropy constant Ku. These factors lead to films having less than desirable parametric performance. Even under the best conditions where epitaxy is favored, there is still inherent mismatch between hexagonal cobalt and cubic chrome lattice. Lack of good epitaxy and subsequent poor growth of the magnetic layer can result in formation of a large amount of imperfections such as dislocations, stacking faults and other irregularities in the crystalline structure which will reduce the Hc potential for a given alloy composition and Pt content.
If the magnetic crystallites contain a high amount of imperfections, the uniaxial crystal anisotropy energy, Ku, of the media will decrease drastically. When a CoCrTa layer is inserted between the CoCrPt based alloy and Cr according to the prior art, it apparently helps the perfection of the CoCrPt based alloy to some extent, but the prior art still has to resort to high Pt containing alloy to raise the Hc. High platinum costs more, reduces the magnetization Ms of the film and hence requires a thicker film to obtain a given magnetic remanence-thickness product (MrT), and also causes more lattice imperfections in the cobalt HCP structure. At above 10% platinum content, there is increased chance of producing FCC (face-centered cubic) crystal structure which will reduce the Ku value as compared with the HCP structure by more than an order of magnitude. Hence adding an excessive amount of Pt in an attempt to raise Hc will defeat the purpose of the benefit of Pt.
A further problem in advanced media is thermal stability i.e. the superparamagnetic effect described earlier. As the volume of magnetic material decreases by virtue of both the decrease in layer thickness and decrease in grain size, the thermal energy of the magnetic particles may exceed the magnetic energy, such that the magnetic grains may switch randomly in an unacceptably short time frame, causing loss of data. As can be seen from the foregoing discussion, as density increases, necessitating smaller grain size for noise considerations, maintaining high thermal stability is difficult.
As is known, the thermal stability is related to the value of KuV/kT, where Ku is the uniaxial crystal anisotropy energy, V is the volume of the magnetic switching unit, k is the Boltzmann constant and T is the temperature of operation of the media. A larger value of this quantity indicates a greater thermal stability. The desire to reduce grain size to reduce noise thereby reduces the value of V, leading to the decrease in thermal stability noted above. The value of Ku is related to the alloy composition and the perfection of the crystal structure of the HCP alloy. In practice, Ku never achieves its full theoretical value because of the presence of crystalline defects, many of which are geometrically necessary to accommodate interfacial lattice misfit. There is always some mismatch which, although may be made reasonably small in one direction cannot be made sufficiently small in both directions. The difficulty arises in growing a cobalt HCP structure on a BCC chrome underlayer in that misfit dislocations or stacking faults will occur as a mechanism to relieve strain.
While the prior art solution of using an intermediate layer between the underlayer and the magnetic layer(s) absorbs some of the defects, and essentially provides a transition between the BCC and the HCP structures, there is still typically sufficient mismatch between the intermediate layer and the magnetic layer such that the crystal imperfections are significant, resulting in a lowered value of Ku and hence decreased thermal stability.
What is needed is a thin film recording media having a high thermal stability while achieving low noise. The media should be capable of being manufactured in a cost effective manner in a high volume production environment.
An improved thin film recording media structure is described. One embodiment of the invention comprises the use of an underlayer such as chrome or chrome alloy and an intermediate layer that forms bi-crystals on the underlayer. In this way, the underlayer can be grown to a sufficient thickness so as to form a desirable texture, such as the (200) texture. The intermediate layer forms bi-crystals on each of the grains of the underlayer, thereby providing a small grained template for the subsequent layer.
In a further aspect of the present invention, two underlayers are used. The first underlayer is designed to match the underlayer and will have some mismatch in lattice parameter from the underlayer. Preferably, the first intermediate layer is the above-described layer forming bi-crystals. The second intermediate layer is designed in particular to match to the ultimate magnetic layer. However, the lattice match of the second intermediate layer is reasonably close to that of the first intermediate layer. While some defects in crystalline structure may be present at the interface of the first and second intermediate layer, the number of such defects is reduced as compared to the number of defects between the first intermediate layer and the underlayer. Therefore, the second intermediate layer can grow relatively defect free to present a high quality template for growth of the magnetic layer.
After deposition of the first and second intermediate layers, a magnetic layer is deposited on the second underlayer. Because the second underlayer has been chosen to match the magnetic layer, there are relatively few defects, resulting in a high quality structure in the magnetic layer and hence increased Ku. In preferred embodiments, one or both of the intermediate layers is non-magnetic. In any event, even if such layers are magnetic their thickness is generally small so that the contribution of these layers to the magnetic moment is small. In embodiments of the present invention, a single magnetic layer may be used, or multiple magnetic layers, including so called laminated structures such as those described in the background section, may be used.
In a further embodiment of the present invention, a layer such as the first intermediate layer described above may be used as a spacer type layer in a laminated magnetic layer structure.
Additional embodiments and other features and advantages of the present invention will become apparent from the detailed description, figures and claims which follow.